Tri-level interferometer

ABSTRACT

Disclosed is an interferometer having three mutually parallel levels, each having a plurality of reflective stripes which divide light entering the interferometer into two groups of parallel beams to permit the phase difference between the beams to be determined by the position of the center level relative to the outer levels.

JJU-LODI Ob-uZ-YZ UR 396590919 United States Patent 3,659,919 Williams May 2,9972

54 TRI-LEVEL INTERFEROMETER FOREIGN PATENTS OR APPLICATIONS [72] lnventor: Charles Sumner Wllllams, Dallas, Tex. 665,978 9/1938 Germany ....350/l63 [73] Assignee: Texas Instruments Incorporated, Dallas, 58,896 2/ I953 Germany ..350/163 Primary Examiner-David Schonberg [22] Filed; D9 2 9 3 Assistant Examiner-Toby l-l. Kusmer Attorney-Samuel M. Mims, Jr., James 0. Dixon. Andrew M. [21] App 783,337 Hassell, Harold Levine. John M. Harrison. Melvin Sharp and John E. Vandigritf [52] U.S.Cl ..350/166, 356/1 12 [51] Int. Cl ..G02b 1/10, GOlb 9/02 [57] ABSTRACT [58] Field at Search ..350/163-166; Disclosed is n in e rome er ha ing three mutually parallel 35 1 12 levels, each having a plurality of reflective stripes which divide light entering the interferometer into two groups of parallel 56] References Cited beams to permit the phase difference between the beams to be 3,410,626 ll/l968 determined by the position of the center level relative to the outer levels.

2 Claims, 2 Drawing figures ELECTRO- MAGNETIC DRIVE ClRCUlTRY Patented May 2, 1972 2 Sheets-Sheet 1 ELECTRO- MAGNETIC T DRIVE CIRCUITRY QOCO/V 34 so 66 20 as 30 INVENIOR CHARLES SUMNER WILLIAMS ATTORNEY Patented May 2, 1972 2 Sheets-Sheet 2 MIRROR MOTION TRI-LEVEL INTERFEROMETER This invention relates to improvements in optical devices and more particularly to improvements in interferometers.

Interferometers are optical devices which divide light incident thereon into two or more mutually comparable beams, and are useful in such applications as performing spectral analysis, or determining small linear displacements, spectral fine structure, and indices of refraction. The interferometer of the invention, although useful as any ordinary interferometer, is particularly intended for use as a spectrometer interferometer for analyzing light to determine the component wavelengths present and the magnitude of each component.

Of primary concern in the design and construction of spectrometer interferometers is the bandwidth of the light frequencies over which operation is intended. Commonly, interferometers, such as the Michelson interferometer, depend on the transmission and reflectance characteristics of a beamsplitter used within the interferometer. Since the efl'ects of reflectance from the beamsplitter are different from the cf fect of the transmittance through it, except, perhaps, within a very limited frequency band, obtaining such interferometer which is operable over a wide frequency range (for example, of wavelengths from 5 to 30 microns such as would be encountered in applications involving infrared radiation from thermal sources such as the earth) is virtually impossible.

In attempts to achieve an interferometer operable over a wide bandwidth, use of grating beamsplitters has been proposed. Grating beamsplitters avoid the above problem of transmitting and reflecting various wavelengths differently because the grating divides the wavefront rather than the amplitude of the impinging beam. For example, a grating beamsplitter and interferometer design are described in copending application, Ser. No. 757,371, filed Sept. 4, 1968, now abandoned by Charles sumner Williams (the inventor herein) and Lee R. Reid entitled Thin Film Wavefront Dividing Beamsplitter," (TI2674), which application is assigned to the assigneeof the present application and incorporated by reference herein. Although the advantages and advancements of the beamsplitter and interferometer of the above-named copending application are apparent, the present invention represents an even further improvement because it can be used when the degree of coherence of the light to be analyzed is very small. Coherence, which is important in the operation of an interferometer, is generally understood to be the phase correspondence of the light waves at various points in a beam. This coherence can be visualized by considering two points in a light beam, each disposed in a plane perpendicular to the direction of travel of the beam. If a vector representing the electromagnetic character of the light is constructed at each point, and the two vectors correspond in phase, except for a constant phase difference, the light is said to be spatially coherent." The farther apart the points can be taken in the plane without destroying the correspondence between the vectors, the higher the degree of spatial coherence of the beam. If, instead, the points are taken along a line in and parallel to the path of the beam, and the electromagnetic vectors correspond, the light is said to be time coherent. Thus, light from a laser source is considered to have a high degree of coherence, since the electromagnetic vectors correspond everywhere along the length, height, and width of the beam. On the other hand, light from the thermal source, such as an incandescent lamp or the earth, has a low degree of coherence since the electromagnetic vectors neither correspond in the direction of travel of the beam nor in a, plane perpendicular to it. Therefore, to analyze a light beam having a low degree of coherence is much more difficult than one having a high degree of coherence, because finding corresponding Jight components at spaced points in a beam of low coherency is more difficult.

It is, therefore, an object of the invention to provide an interferometer which can be used in the infrared wavelength of the electromagnetic spectrum.

It is a further object of the invention to provide an interferometer which can be used in conjunction with light sources having a small degree of coherence.

Other objects, features, and advantages of the invention can be readily seen from the appended claims when read in conjunction with the following detailed description and the attached drawings in which:

FIG. I is a perspective view of a partially cut-away beamsplitter in accordance with the invention, and

FIG. 2 is a side view of the beamsplitter of the invention, which has been cut away to show its operation and the relationship between its parts.

In accordance with the invention, an interferometer is provided which has three parallel levels, each level consisting of a grating of alternating reflective and transmittive portions. (The term grating" as used herein is understood not to mean exclusively the type of grating often referred to as a "diffraction grating" which acts to disperse the colors in a multichromatic beam). The center level functions as a wavefront dividing beamsplitter which is movable toward and away from the other two levels to control the phase of the two beams formed by the grating of the first level.

Referring now to FIG. I, it can be seen that the interferometer of the invention has three levels, hereinafter referred to, respectively, by reference numbers 100, 200, and 300. Each level is constructed upon a frame, designated respectively by reference numbers 10, 20, and 30, and is configured into a grating pattern of stripes, reflective from both sides. The stripes may be formed, for example, by vapor deposition on a pellicle such as the pellicle described in copending patent application, Ser. No. 497,294, filed Oct. 18, I965, by Lee R. Reid, entitled "Beam Splitter," said application being assigned to the assignee of the present application and incorporated herein by reference. The pellicle as therein described may be of a nitrocellulose material and of a thickness of approximately 0.5 micron. Such reflective stripes may be, for example, of gold, silver, aluminum or other similar reflective material, but of particular importance is that the reflecting surfaces be optically flat, representing as true a plane as possible. To be understood is that the use of a pellicle as a base for mounting the stripes is not an absolute requirement, as the gratings could be constructed in a number of different ways; for example, the grating may be etched through steel sheets polished to optical flatness and coated with a reflective material, or a reflective material could be deposited upon a pre cut transparent substrate. Since interferometers deal with differences in distances the size of a wavelength of light, it can be appreciated that very slight defects in the optical flatness can introduce large errors in the output. It has been found convenient to define the term optically flat" as a flatness having no variations from a true plane greater than approximately one wavelength of green light; however, this requirement may be relaxed for particular uses wherein such close accuracy is not required.

The levels are mounted and supported in a given relation to one another by frames 60 and 61, constructed, for example, of aluminum, Bakelite, formica, or other suitable material. Frames 60 and 6I are identical, having grooves, 62 and 63, 64 and 65, respectively, cut therein to receive a portion of the outside beam splitter levels and 300. Between the grooved portions of frames 60 and 61, flat recessed portions 66 and 67 are, respectively, formed to accommodate the inner level 200, allowing it to travel freely in the direction perpendicular to its own plane and also perpendicular to the plane of the other two levels. The inner level 200 may be moved by a connection (not shown) to a wonn gear 50, driven by a driving means 51. Such driving means 51 may be activated by an electromagnetic drive circuit 52 which may in turn respondto a computer program or other external stimulii. Although not shown, a driving means, perhaps a similar worm gear mechanism, may be utilized on the opposite side of the center level to ensure that the inner level 200 moves uniformly with respect to the outer levels 100 and 300.

For a detailed description of the operation of the interferometer, reference is made to FIG. 2. The three levels of the beamsplitter are mounted in mutually parallel planes, disposed at an angle, I, to the oncoming light beam to affect proper reflection within the interferometer. Light enters the The other two levels, 200 and 300, are constructed similarly to the first level 100 except the reflective stripes deposited on them are of widths operation of the interferometer, three examples of the possible light paths through the interferometer are shown in FIG. 2. One portion of one of the beams passing through the first level 100 is shown at the uppermost part ofthe drawing, and is denoted at its edges by the characters A, B, and C. A portion of the light beam passes through the transparent portions 120 and 21a of the first and second levels, respectively, to impinge upon the reflective stripe 32 on the third level 300.

transmitted through transparent area 140 of the first level impinges upon the front on the second level 200, and is A composite light beam is shown in the bottommost portion of the drawing of FIG. 2, and the center portions of the beams respectively corresponding to beams A-B and BC' are denoted by the characters A" and B". The light beam, entering as a single beam, passes through transparent area 160 and processed by a light processing devices being well known in the art.

As above mentioned, the degree of coherence of the light to 200, between the gratings of the first level, beyond that distance,

levels, reduce the effintroduce extranetherebetween were not proper.

It can be seen from the above discussion that the grating size but must be constrained within somewhat arbitrary, but are constrained by the spacings of the gratings and the distance the center level must move.

Although the invention has been described and illustrated with a certain degree of particularity,

What is claimed is:

1. An interferometer comprising:

a first optically fiat thin film transparent pellicle disposed at a preselected angle to an oncoming light beam,

a first plurality of optically fiat reflective stripes affixed to said first pellicle defining a grating of sequential reflective and transparent areas,

a second optically flat thin film transparent pellicle disposed in a plane parallel to and movable toward and away from said first pellicle,

mitted through said second pellicle is transmitted through to also impinge upon said detector. and

it is understood that the 3,659,919 6 an electro-magnetic drive means for moving said second 2. The interferometer of claim I wherein said reflective pellicle in a direction parallel to said first and third pellistripes are selected from the group consisting of gold; silver, cles, whereby movements of said second pellicle' will and l'f'r p bein! form l y cause the phase between said two emerging beams of light vapor deposmon' impinging upon said detector to be changed. 5 

1. An interferometer comprising: a first optically flat thin film transparent pellicle disposed at a preselected angle to an oncoming light beam, a first plurality of optically flat reflective stripes affixed to said first pellicle defining a grating of sequential reflective and transparent areas, a second optically flat thin film transparent pellicle disposed in a plane parallel to and movable toward and away from said first pellicle, said second pellicle being spaced apart from said first pellicle by a distance less than about 1,000 times the distance between the stripes of said first pellicle, a second plurality of optically flat reflective stripes affixed to said second pellicle each disposed so as to divide light transmitted through said first pellicle into two beams, one transmitted through said second pellicle and the other reflected back upon said plurality of reflective stripes of the first pellicle so that the beam reflected from said plurality of stripes on said first pellicle is transmitted through said second pellicle, a third optically flat thin film transparent pellicle in a plane mutually parallel to the planes of said first and second pellicles, said third pellicle being spaced apart from said second pellicle by a distance less than about 1,000 times the distance between the stripes of said second pellicle, a third plurality of optically flat reflective stripes affixed to said third pellicle in a relationship such that said beam of light which was transmitted through said first and second pellicles is reflected by one of said third plurality of stripes onto one of said second plurality of stripes, and the reflected beam from said second plurality of stripes is transmitted through said third pellicle upon a detector, and said light beam which was reflected from said second plurality of stripes, said first plurality of stripes, and transmitted through said second pellicle is transmitted through said third pellicle and to also impinge upon said detector, and an electro-magnetic drive means for moving said second pellicle in a direction parallel to said first and third pellicles, whereby movements of said second pellicle will cause the phase between said two emerging beams of light impinging upon said detector to be changed.
 2. The interferometer of claim 1 wherein said reflective stripes are selected from the group consisting of gold, silver, and aluminum, said stripes being formed on said pellicle by vapor deposition. 